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mircea nicolae
31-May-2013, 10:54
Hello,

I would like to change my Ground Glass / Fresnel that came with my Shen Hao TFC45 IIB camera.
https://www.badgergraphic.com/store/cart.php?m=product_detail&p=3165

I found a bright Ground Glass / Fresnel on Ebay that might work.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=281105774191&ssPageName=ADME:X:eRTM:US:1123


However, I am a bit confused by some technical questions.
I made diagrams for three types of setup - Ground Glass only, Ground Glass and plastic Fresnel, and Yanke Fresnel.

961369613796138


1.Considering my drawings, where is the sheet film plane? Is it:

a. on the plane of the Glass holder (number 5 on my drawing)?
b. or on the plane of the Fresnel holder (number 6 on my drawing)?


2. My guess is that if the film plane is on the plane of the Glass holder (number 5 on the drawing), that means that the Yanke Fresnel / GG I would like to buy will displace the focus plane by 1.5 mm towards the lens, because the image is formed on the matt layer facing the lens. Is this so?

If it is, it would mean the brighter Yankee Fresnel / GG would actually be less useful than what I already have installed on the camera.
Please tell me if I am missing something.


3. On my Shen Hao camera, the dimensions of Ground Glass are different from the Fresnel.


the Ground Glass is - 127 mm x 101 mm
the Fresnel is - 123 mm x 101 mm

Each of them has a distinctive slot into which they are placed. The guy on Ebay seems to be completely confused as to the need to have different measurements on the Protective Glass and on the Fresnel.

a. If any of you have bought and installed a Yanke Fresnel / GG on a Shen Hao, what is your experience with this?
b. Does the Protective Glass come in a wider size than the Fresnel, so that it actually fits the camera?


4. When you received the Yanke Fresnel / GG, did you have to peel off some layer of plastic from the Fresnel?



I know I am asking many questions, so any info would be appreciated.


Thanks.

Nathan Potter
31-May-2013, 16:52
I always use the fresnel behind the ground glass so that the frosted side of the glass sees the image unperturbed from the lens. The ground surface needs to be exactly in the plane of the film emulsion when the holder replaces the glass.

I think you can put the fresnel in front of the GG but note that there will be a perceptable change in focal plane due to the refraction occuring in the plastic - the extent of which is calculated by Snell's Law. That shift will not be seen by the film since the fresnel is removed during exposure. Also note that under that arrangement the path length of far off axis rays have a progressively longer path to travel through the plastic as you go further toward the edge of the format so the point of focus varies from center film to edge. Whether this is all very critical or not depends largely on the aperture you use. So with small aperture (large DOF) it may be irrelevant.

Look up Snell's Law and do a simple calculation to see the effect of fresnel between lens and frosted glass surface.

Nate Potter, Austin TX.

Cletus
31-May-2013, 19:28
Without reading this whole thing, I'll give you my experience with the excellent (or at least very good) Yanke Fresnel. I used it with a Shen Hao HZX too a d worked brilliantly. I removed the GG, inserted the fresnel first (closest to the lens) with the "rough side" toward the lens. Then the plastic GG, then the cover glass purchased from Steve Hopf.

Never had any apparent focus issues with the 1.5mm offset and the inexpensive Yanke Fresnel was bright enough to focus without a darkcloth in anything but full sun, just about. You can hardly go wrong. Just don't unscrew and re screw the little GG holder screws on the camera back too much or you won't be able to tighten them properly.

Nathan Potter
2-Jun-2013, 11:37
This is an interesting question that I've sometimes thought about but not investigated. What is the actual effect of placing a plastic fresnel between the lens and the GG? That is aside from displacing the plane of best focus when inserting the film holder.

One obvious effect is the longer optical path for far off axis rays due to refraction. A key reason not to use behind the lens filters - at least on wide angle lenses. The effect of using the fresnel between lens and GG is shown diagramatically below. The top view shows a hypothetical lens with a 90 degree field of view and well corrected for a flat field, without interference from the fresnel. The points of best focus are represented by the dots.

The lower drawing represents what happens to the points of best focus radially when a fresnel in placed between the lens and the GG frosted surface. Red line bb is the same length as red line aa (well I didn't draw it quite the same length) but due to refraction falls short of the GG surface. The question to ask is how serious is this effect in the real world. It is easily determined

http://farm4.staticflickr.com/3708/8927404214_b4f078bff7_m.jpg (http://www.flickr.com/photos/argiolus/8927404214/)
FRESNAL-2 (http://www.flickr.com/photos/argiolus/8927404214/) by hypolimnas (http://www.flickr.com/people/argiolus/), on Flickr

The degree of bending of the edge ray is determined by Snells Law and we find that for an incoming edge ray at 45 degrees from the vertical (1/2 of the 90 degree FOV) the refracted ray is bent 28 degrees from the vertical assuming the index of the fresnel is about 1.5 (close to plexi). Hmm, more than I would have thought.

Just for example let's assume the fresnel is 1 mm (1000 Ám) thick. Constructing the appropriate right triangle we can use cos 28 degrees = 1000 Ám X extended bb. Computing this we get the new path length of bb to be 1105 Ám - meaning the point of best focus has slipped 105 Ám forward from the flat field position in the top drawing. ( A little less than 1105 when taken orthogonal to the GG surface). Doesn't seem to be very much.

If we check the DOFocus for a lens that might be used, DOFocus = COC X aperture, we can use common settings of COC = 50Ám and N = f/22 for pretty critical work. This yields a DOFocus of 1100 Ám (1.1 mm), really much larger than the 105 Ám displacement due to refraction. The effect can be easily ignored for even pretty critical work. You can draw your own conclusions.

Nate Potter, Austin TX.

Doremus Scudder
3-Jun-2013, 00:02
Short answer: Unless the camera is designed to have the Fresnel screen installed between lens and ground glass, i.e., unless there is a slot the proper depth for the Fresnel to sit in, so it does not displace the ground glass back by its entire width, then you should install the Fresnel behind the ground glass, i.e., between your eye and the ground glass.

Not doing so will result in focus shift and unsharp negatives.

Some cameras are designed to have the Fresnel installed between lens and ground glass. These all have provision for correct installation.

Contact your camera's manufacturer if you are not certain.

Best,

Doremus

mircea nicolae
4-Jun-2013, 01:38
Thank you for your replies.


@ Nathan Potter

Just to make things more understandable for myself, I drew a diagram of the setup you talked about.

96375

If I understand things correctly, in this case, the Fresnel works as a loupe that magnifies only the image that was already formed on the GG.
Also, if the GG is properly aligned on the film plane line, there is no Fresnel displacement anymore, as it sits behind the GG and does not modify in any way optically the light between the lens and the matt surface of the GG.

Any distortions that the Fresnel might have are no longer caught on film. As any magnifier loupe, the Fresnel behind the GG is only a passive component in this setup.



@ Cletus

With the same idea of making things clearer, at least to myself, I drew a diagram of your setup as well.

96376

Here, again, if I got it right, the setup is different from what the manufacturer intended.



Installation Guide:

Remove your original ground glass, put the ultra slim Yanke Fresnel ground glass matt surface toward the lens, and then put the protection glass behind the ground glass. http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=281105774191&ssPageName=ADME:X:eRTM:US:1123

In the indication above, I take the matt surface to be the opposite of the etched part of the Fresnel.
In this case, it is what the producer considers to be 'the ground glass'.

So basically the producer says one should install the matt surface towards the lens and the etched side towards the photographer, which is exactly the opposite of what Cletus did.

Again, if I get it right, he did this to avoid focus shift due to the fact that the setup proposed by the manufacturer places the GG in front, thus displacing the matt image-forming surface forward, because the plexi is 1.5 mm thick.

To conclude, Cletus avoided focus shift due to the thickness of the plexi, by putting the matt image-forming surface in the middle, between the so-called glass protector at the back and the etched surface of the Fresnel at the front. The idea was to have the matt side sitting as close as possible to the film plane.

However, focus shift due to Fresnel optics was not avoided. Or maybe it was, if the Shen Haos we are all using are optimized for the kind of small distortion the Fresnel brings into the equation, which we will never know.



@ Doremus Scudder

Which brings me to the last post.

I would gladly bring all this up during a discussion with the manufacturers, either of the Yanke, or of the Shen Hao.
However, in my experience, they either speak very poor English, or they could not bother to actually respond.

In Yanke's case, a discussion was brought to a halt when the Yanke person could not understand my question regarding the construction of the Fresnel.
I had just bought another plastic Fresnel + GG combination from another seller, and was quite disappointed to find that what they called a Ground Glass was actually a piece of self-adhesive plastic. Basically, they had taped some transparent stuff on the plastic Fresnel. When installed, it did not have the right transparency, so even at best focus, the image was very blurred.

In order to avoid the same, I asked the same question to the Yanke people, but they were not able to understand what I was asking.
After 10 emails, I gave up when the guy said 'he does not understand my meaning'.

Maybe I will go back to that conversation, but that's the way it went so far.

As for Shen Hao, I ordered a custom camera from them. When it came back with some finishing problems, they kind of told me politely to mind my own business.
So, in my experience, these kind of problems are the kind you figure out on your own.

Or with the help of people from the forum, which I am grateful for.


To wrap it up, my camera is the kind where the Fresnel is supposed to be in front of the GG. There are special slots for each, with differing measurements.

the Ground Glass is - 127 mm x 101 mm
the Fresnel is - 123 mm x 101 mm


Basically, you cannot switch them and avoid the optical distortions of the Fresnel.

The Yanke people are manufacturing their items, and I was thinking to make a custom order of the GG and Fresnel for a switched setup, just like Nathan mentioned.
However, I am still considering the headaches the language barrier will produce once I will try to explain what I want.


In the end, I am thinking to order a regular Fresnel first, GG second setup, as in the case of Cletus.



One last question for Nathan.

Do you think that the Fresnel-first kind of distortion would have a great impact on images that are printed on a 100 x 80 cm format?
From your calculations, would it be possible to translate the difference between the undistorted, simple GG setup, and the shifted, Fresnel + GG setup, in meters?

Say, if I have a 150mm Sironar lens, and I am trying to get the best focus of:
1. an object not farther than 14 meters away, like a building facade
2. or an object that is quite far, like an urban situation with crossroads, where the central object is 50 meters away

What would that displacement by the Fresnel account for in meters in this situation?